Midday Stomatal Closure in Mediterranean Type Sclerophylls Under Simulated Habitat Conditions in an Environmental Chamber II. Effect of the Complex of Leaf Temperature and Air Humidity on Gas Exchange of Arbutus unedo and Quercus ilex

Shrubs of the Mediterranean sclerophyllous species Arbutus unedo and Quercus ilex were studied under simulated habitat conditions in an environmental chamber. Temperature, humidity, and light intensity were altered stepwise to simulate diurnal changes in conditions similar to those measured in an evergreen macchia in Sobreda, Portugal. Leaves were enclosed in cuvettes which reproduced the growth chamber climate and which allowed measurement of gas exchange. Increasing atmo- spheric stress in the form of higher temperature and lower humid- ity on successive days gradually results in midday depression of transpiration rate and net photosynthesis rate of leaves due to midday stomatal closure.